1. The purpose of this core is to provide a facility for the propagation of xenografts of pediatric solo tumors, for use in all projects. The standard model will comprise tumor growing in the subcutaneous space of female severe combined immunodeficient (SCID) mice. 2. Coordinate tumor transplantation and make available tumor-bearing mice for biochemical and pharmacokinetic studies as required. 3. Undertake drug evaluation studies using standardized protocols. Data will be accessed directly into a microcomputer, and information distributed on a regular basis to individual project leaders, and to the Biostatistics Core for analysis. 4. Establish models of minimal residual disease for evaluating both cytotoxic and novel approaches to tumor eradication (e.g. gene therapy/antisense). 5. Develop models of disseminated disease as a secondary screen for drug evaluation. 6. Develop disseminated disease models with luciferase reporters for DNA damage, hypoxia, to monitor tumor growth, spread and drug-response using non-invasive techniques (Xenogen system). 7. Maintain and characterize human xenografts, and maintain frozen stocks. 8. Provide services (e.g. blood collection, 1 to 5 day infusions in mice). 9. To evaluate transgenic tumor models where requested. ) A program 'working committee' will serve to prioritize the order for evaluating new agents and strategies derived from individual projects, and for allocating resources.